The most popular fat burners are thermogenic fat burners, which contain small amounts of ephedrine (ma huang su) => acts as a stimulant and increases metabolism. Ephedrine is actually a banned substance, but you can still buy ephedrine-based fat burners on the internet, in locker rooms etc. Athletes use fat burners in order to lose body fat or just to increase metabolism. But ephedrine-based supplements are actually quite dangerous and can cause heart problems, stroke and occasionally even death. And even if you buy a fat burner without ephedrine, it might contain something similar with similar side effects. Something that just isn't illegal YET. I'm not saying that you should stay away from fat burners at all, but you should not buy the next best product and you should definitely consult your doctor first. Find more sports nutrition related topics at nutrition main.